Recently, laser beam machining has been also applied to the field of welding. Since laser beam machining is conducted by a heat source of extremely high energy density, thermal effect is small in the portions other than a portion irradiated with lasers, so that minute portions or parts required high accuracy can be welded under the condition of low strain. Further, lasers not attenuate in atmosphere, and generates no X-rays. Therefore, laser welding can be conducted in atmosphere, and there is no restriction on the shape and size of members to be welded.
However, as shown in FIGS. 7(a)-7(d), when a copper member 1 was put on an iron member 3 and the copper member 1 was irradiated with lasers 5 to weld the copper member 1 and the iron member 3, the copper member 1 melted first as shown in FIG. 7(a), and melting of the copper member 1 progressed as shown in FIG. 7(b). When the melting zone of copper member 1 had reached the iron member 3, a portion of the iron member 3 irradiated with lasers 5, was scattered. Accordingly, a hole was formed in the members as shown in FIG. 7(c). In the drawings, numeral 6 is a metal layer which was melted by lasers, and numeral 61 is a metal particle which was scattered.
In order to prevent the iron member 3 from scattering, laser input (referred to as laser irradiating power, hereinafter) was reduced. Then, a problem arose in which the copper member 1 did sufficiently melt so that welding could not be conducted.
The inventors earnestly investigated the cause of the aforementioned problem. As a result, they found that the cause was the laser reflection factor of each metal member to be welded.
It is a primary object of the present invention to positively weld different kinds of metals, paying attention to the laser reflection factor.